geochemical behavior of lead in an alfisol and an ultisol ... · pdf filean alfisol was...

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Copyright© 1996, CRC Press, Inc. — Files may be downloaded for personal use only. Reproduction of thismaterial without the consent of the publisher is prohibited.

Journal of Soil Contamination, 5(1): (1996)

Geochemical Behavior of Leadin an Alfisol and an Ultisolat High Levels of Contamination

Jeffrey L. Howard and Grazyna SledzinskiDepartment of Geology, Wayne State University, Detroit, MI 48202

ABSTRACT : The behavior of Pb in the A and B horizons of an Alfisol from Michigan and anUltisol from Virginia was studied to determine the effects of “shock” loading. Combinedsequential extraction-sorption isotherm analysis (CSSA), a relatively new and little testedmethod, was used in the study. After spiking to simulate severe contamination (~3000 to60,000 mg/kg), CSSA revealed unexpectedly high levels of exchangeable Pb in the A horizonof the Alfisol and in both horizons of the Ultisol, and showed that the sorption capacities of thephases commonly responsible for fixation of Pb at low to moderate levels of contamination wereexceeded. Carbonate sorbed the bulk of the Pb in the Alfisol B horizon and has a high sorptioncapacity in both soils, despite the presence of other phases with a strong affinity for Pb. Thus,when shock loading occurs (e.g., at a shooting range or dump sites), the highly contaminated Ahorizons of both soils are expected to pose a serious toxic hazard to humans, and groundwatercontamination is possible in association with the Ultisol. CSSA proved useful for determiningthe sorption capacities of the individual phases while together in a natural soil system andtherefore is a valuable method for predicting the attenuation capabilities of soils.

KEY WORDS: sequential extraction, sorption isotherms, soil contamination, heavy metals.

I. INTRODUCTION

Lead is one of the most commonly encountered heavy metals in contaminatedsoils, and it is the one most often found at hazardous-waste sites (Sims et al., 1986).Lead is a toxic heavy metal of environmental concern because of its tendency tobioaccumulate and damage the skeletons, internal organs, and nervous systems ofanimals (Luoma, 1983; Chaney et al., 1988; Gunn et al., 1988). Lead also mayenter the food chain by way of plants, but this is generally considered to be a muchless significant pathway than animal uptake (Kabata-Pendias, 1984; Alloway andAyres, 1993).

Many studies have been made of soils containing low to moderate levels of Pbcontamination. This is typically the result of anthropogenic enrichment via agricul-tural amendents (e.g., sewage sludge) and airborne deposition in urban settings. In

2


both cases, the levels and rates of Pb loading are low. Few studies have been madeof severely contaminated soils. High levels of contamination may result fromairborne deposition downwind from smelters, by decomposition of Pb shot atshooting ranges, and from refuse at battery recyling plants or dump sites. Airbornedeposition rates are probably low in general, but published (Jorgensen and Willems,1987) and proprietary data indicate that the breakdown of Pb shot occurs rapidlyand causes “shock” loadings. Presumably, this also occurs when battery partsdecompose, but no published studies of this phenomenon are known to us. It isimportant that shock loadings at high levels be studied because the behavior of Pbis likely to be quite different from that when loading rates and levels are low.

The purpose of this investigation was to characterize the behavior and fate of Pbat high levels of contamination under conditions of shock loading. Combinedsequential extraction-sorption isotherm analysis (CSSA), a relatively new and littletested method, was used in the study. An Alfisol was contrasted with an Ultisol toexplore a wide range of soil characteristics. Following the method of Salim et al.(1993, in press), the soils were first spiked with solutions containing high levels(1000 to 8000 mg/l) of Pb and sorption isotherms were constructed for the bulk soilsamples. The soils were subsequently subjected to sequential extraction analysis,and sorption isotherms were then constructed for each individual targeted form.This paper discusses the Pb partitioning and sorption capacities of whole soils andthe individual phases composing them. Only shock loadings by Pb are considered;the effects of prolongated accumulation at low loading levels, and of competitionwith other metals, were not studied.

II. PREVIOUS WORK

Lead is usually a minor constituent of Earth materials, the average concentrationin uncontaminated soils being less than about 50 mg/kg (Nriagu, 1978; Shackletteand Boerngen, 1984; Adriano, 1986; Davies, 1990). Previous studies show that lowto moderate levels of Pb contamination (~100 to 1000 mg/kg) are common in soilsas a result of the addition of agricultural amendments (Page, 1974) and airbornedeposition in urban settings (Motto et al., 1970; Ward et al., 1977; Parker et al.,1978; Harrison et al., 1981; Gatz et al., 1981; Miller et al., 1983; Miller andMcFee, 1983; Gibson and Farmer, 1986; Yassoglou et al., 1987). High levels ofcontamination (>1000 mg/kg) have been documented in soils affected by smelting(Marten and Hammond, 1966; Buchauer, 1973; Lagerwerff et al., 1973; Wagnerand Siddiqi, 1973; Temple et al., 1977; Beyer et al., 1984), by Pb ammunition atshooting ranges (Jorgensen and Willems, 1987), and by battery recycling plants(Elliott et al., 1989). The average concentration of soil Pb at hazardous waste sitesis about 300 mg/kg (Eckel et al., 1985), but levels ranging up to 466,000 mg/kghave been reported (Sims et al., 1986).

Lead and other heavy metals in soils generally can be found in a variety of formsexhibiting different degrees of bioavailability and mobility. The geochemical

3


partitioning may be determined by sequential extraction analysis (Tessier et al.,1979). This method has been widely used (Martin et al., 1987; Forstner, 1993), butprevious studies are mainly limited to soils and sediments containing low tomoderate concentrations of Pb. We know of only one published study dealing withseverely contaminated soil (Elliott et al., 1989). Sorption isotherm analysis is auseful technique for characterizing the adsorption characteristics and capacities ofEarth materials (Kinniburgh, 1986). This technique has often been used to examinethe sorption characteristics of Pb (Scrudato and Estes, 1975; Griffin and Au, 1977;Zimdahl and Skogerboe, 1977; Harter, 1979; McKenzie, 1980; Farrah et al., 1980;Aualiitia and Pickering, 1987), but these studies have been limited to studies ofwhole soils, or individual soil components in isolation. Salim et al. (1993) foundthat by combining the sequential extraction and sorption isotherm methods, it ispossible to characterize the sorption characteristics of individual phases whiletogether in a natural system. However, their study was restricted to one particulartype of sediment.

III. MATERIALS AND METHODS

The Alfisol samples used in this study were collected at a rural site along I-96 nearHowell, MI, about 60 km northwest of the city limits of Detroit. The sampling siteis 100 m downwind from the highway in an open pasture. A carbon steel bucketauger was used for sampling. The Ap and Bt horizon samples represent the 0 to15 cm and 31 to 45 cm depth intervals, respectively. The Ultisol samples werecollected 350 m north of the junction of state highways 619 and 662 in ruralDinwiddie County, VA. Samples were obtained from a hand-dug pit. The Ap andBt horizon samples represent the 0 to 20 cm and 21 to 42.5 cm depth intervals,respectively. The soil samples were air dried and gently ground to obtain the<2-mm fraction by sieving. Each sample was then characterized in terms of color,particle size distribution (pipette method), pH (1:1, soil:water), organic mattercontent (Walkley-Black method), Fe-oxide content (determined by measuring Fein step 5 below and converting to weight percent FeOOH), carbonate content(determined gravimetrically by leaching 10-g soil samples with 100 ml of 10%HCl for 24 h), and clay mineralogy. The mineralogical composition of the clayfraction was determined by X-ray diffraction analysis using a Rigaku RU 200rotating anode power diffractometer after pretreatment to remove soluble salts,carbonates, organic matter, and Fe-oxides (Kunze, 1965). The proportions of clayminerals present were estimated using the method of Weir et al. (1975).

After the basic characteristics of each soil were determined, each sample wascharacterized by sequential extraction analysis (as described below) to determinepartitioning at background levels. Subsequently, severe contamination was simu-lated by spiking 2.5-g soil samples with 40 ml of solutions containing the followingconcentrations of Pb: 500, 1000, 3000, 4000, 5000, 6000, and 8000 mg/l. These

4


solutions were prepared using Pb(NO3)2. The samples were mixed on a wristshaker for 7 d. Previous kinetic studies on clayey glacial sediments in Michigan(Salim, 1994) suggest that Pb approaches equilibrium after this time. The Pbconcentrations in these supernatants were used to construct sorption isotherms forthe bulk samples. After spiking and removal of supernatants, the remaining soilsolid components were subjected to sequential extraction analysis. The procedureused is based on the methods of Chao (1972), Tessier et al. (1979), and Miller et al.(1986). Sequential extraction procedures are affected by a certain amount ofnonselectivity and readsorption; hence, the steps are actually operationally definedin terms of the particular phases targeted. The targeted forms, extractants, andequilibrium times, respectively, were as follows:

• Step 1 (exchangeable): 20 ml 1 M MgCl2, 1 h

• Step 2 (carbonate occluded): 20 ml 1 M NaOAc, 5 h

• Step 3 (Mn-oxide occluded): 20 ml 0.1 M NH2OH–HCl + 0.01 M HNO3

30 min

• Step 4 (organically bound): 20 ml 0.1 M K4P2O7, 24 h

• Step 5 (Fe-oxide occluded): 20 ml 1 M NH2OH-HCl + 25% (v/v) HOAc, 4 h

Reagents were prepared from analytical-grade chemicals using distilled-deion-ized water. After each individual step was performed, the supernatants wereobtained by centrifugation, acidified to pH <2.0, and placed in acid-washed polypro-pylene bottles. All extractions were carried out in triplicate, and Pb was determinedby flame atomic absorption spectrophotometry using a Varian AA-1275 instru-ment equipped with a deuterium lamp background correction. Relative standarddeviations of analytical results are ≤20%.

IV. RESULTS AND DISCUSSION

A. Soil Characterization

Southeastern Michigan, where the Alfisol (Typic Hapludalf, soil series unknown)was sampled, is characterized by a humid-temperate (mesic) climate. The soilparent material is sandy calcareous glacial till of Wisconsinan age, and soil genesishas taken place over approximately the last 11,000 years (Hansel et al., 1985). Thesoil is moderately well drained and relatively uniform as a function of depth. Thethickness of the solum is about 75 cm. Table 1 shows that the Alfisol A horizonis sandy loam, whereas the B horizon is sandy clay loam. Both horizons havebrown (10YR) hues. The pH of both horizons is slightly acid. Carbonate and Fe-oxide contents are low, but there is a significant increase in the carbonate levels in

5


TA

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1C

hara

cter

istic

s of

the

Soi

l Typ

es S

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e S

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OH

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rg.

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lay

min

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b

Sam

ple

type

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pH%

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sol

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/45.

356

.827

.315

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.334

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400.

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10 Q

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C3

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or.

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/46.

273

.314

.112

.62.

300.

80.

75N

Dc

Ulti

sol

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5 Y

R 5

/65.

651

.96.

042

.16.

600.

60.

38K

55 C

V30

G9

S5 I

1

Gla

cial

lak

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ds.

d2.

5 Y

R 5

/67.

818

.032

.050

.00.

1525

.60.

74I

61 K

13 C

8 O

18

aD

ry.

bI,

illite

; K, k

aolin

ite; C

, chl

orite

; O, q

uart

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feld

spar

+ h

ornb

lend

e +

cal

cite

; CV

, chl

oriti

zed-

verm

icul

ite; V

, ver

mic

ulite

; G, g

ibbs

ite; S

, sm

ectit

e;Q

, qu

artz

; F

, fe

ldsp

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cN

ot d

eter

min

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dD

ata

from

Sal

im e

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993)

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fou

r sa

mpl

es.

6


the subsoil. Organic matter content is average and decreases with depth. The claymineralogy of both horizons is very similar. The predominance of illite implies thatthe cation exchange capacity (CEC) is moderate to low.

The Ultisol samples are from the inner Coastal Plain province of southeasternVirginia, where the climate is humid-temperate (thermic). The soil parent materialis sandy alluvium of Pliocene age (Carpenter and Carpenter, 1991), and the profileis a very well drained Plinthic Paleudult (Varina Series) with a solum 83 cm thick.Table 1 shows that the quantities of FeOOH in both the A and B horizons are quitehigh, as expected for such an old soil. The presence of carbonate is apparently theresult of artificial liming; Coastal Plain soils are typically thoroughly leached andstrongly acidic. The clay fraction contains mostly kaolinite, chloritized-vermicu-lite, and gibbsite. This implies that the CEC is very low.

B. Background Levels of Lead

The results of sequential extraction analysis (Table 2) show that the Pb in theAlfisol A horizon is associated mostly with the organic and carbonate-occludedfractions, with lesser amounts in the exchangeable, Fe-oxide-occluded, and Mn-oxide-occluded fractions. In the B horizon, most of the Pb is in the organic fraction,suggesting that some downward movement of Pb has occurred as a result ofilluviation. The total background Pb content of the Alfisol is within the range of5 to 22 mg/kg, comparable to the results of Kessler-Arnold and O’Hearn (1989),who reported that the average total Pb content of southeastern Michigan soilsranges from 10 to 18 mg/kg. Background concentrations of Pb in Quaternarysediments of the Great Lakes region are also generally below about 25 mg/kg(Fitchko and Hutchinson, 1979; Konkel, 1979; Mudroch et al., 1988; Salim et al.,1993).

The Ultisol has lower levels of Pb than the Alfisol, especially in the A horizon.Sequential extraction data (Table 2) show that the Pb is mainly in the Fe-oxide-occluded form in both horizons, probably due to the high levels of Fe-oxidespresent. Organically bound Pb has a higher concentration in the A horizon, and thecarbonate-occluded fraction contains higher levels of Pb in the Ultisol B horizon.Overall, the total background levels of Pb are between 4.0 and 10.0 mg/kg. Tidball(1976), Shacklette and Boerngen (1984), and Holmgren et al. (1993) have alsoreported that the Pb content of soils in the Atlantic Coastal Plain is generally low,with concentrations equal to or less than 14 mg/kg.

C. Lead Distribution at High Levels of Contamination

The spiked Alfisol and Ultisol soil samples sorbed large quantities (~3000 to60,000 mg/kg) of Pb (Table 3). The sequential extraction data show that the

7


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10


sorbed Pb in the Alfisol A horizon is mostly in the exchangeable (~68% ofsummed total) and carbonate-occluded (~21% of summed total) fractions. Itwas expected that significant levels of exchangeable Pb would be presentbecause this form typically increases with increasing pollution as other phasesbecome overloaded (Engler, 1980; Forstner and Wittman, 1981; Forstner,1989). However, it is surprising that more Pb is not organically bound becausethis form is prominent at background levels (Table 2) and is often predominantin soil surface horizons (Zimdahl and Skogerboe, 1977; Berrow and Mitchell,1980; Miller and McFee, 1983; Miller et al., 1983; Adriano, 1986; Davies,1990). The association of Pb with carbonate, especially in the Alfisol Bhorizon, is expected because Pb has often been found in the carbonate fractionof calcareous soils (Sposito et al., 1982; Gibson and Farmer, 1984, 1986;Yassoglou et al., 1987; Howard and Sova, 1993). The results are also similarto those of Elliott et al. (1989), who studied a soil containing about200,000 mg/kg Pb and found most of the metal in an acid-soluble (presumablycarbonate-occluded) form. In contrast with Harter (1979), who found a goodcorrelation between Pb sorption and vermiculite content, the differences in thelevels of exchangeable Pb in the A and B horizons of the Alfisol studied hereare probably due to differences in organic matter content because the propor-tions of vermiculite in the clay fractions of both horizons are about the same(Table 1).

Unlike the Alfisol, the distribution of the Pb in the Ultisol is about the samein both horizons (Table 3). The Pb is found mainly in exchangeable (~58% ofsummed total) and carbonate (~30% of summed total) forms. The differencesin the levels of exchangeable Pb in the two horizons correspond better withdifferences in clay content than with organic matter content (Table 1). This isattributable to the low levels of organic matter present. As noted above, theassociation of Pb with carbonate is not surprising, and both clay and carbonatecontent have previously been found to be good predictors of Pb retention bysoils (Korte et al., 1976; King, 1988). More Pb is associated with carbonate inthe spiked samples than at background levels (Table 2), probably because limewas recently added as an agricultural amendment. The presence of lime sug-gests that Pb may be precipitating as a separate carbonate phase, althoughfurther study is needed to document this reaction. Iron-oxide content is usuallya good predictor of Pb retention by soil (Jenne, 1968; Korte et al., 1976; King,1988), probably because oxides have a strong specific sorption affinity for Pb(McKenzie, 1980; Aualiitia and Pickering, 1987). The oxides in the Ultisol(Plinthic Paleudult) studied may have a low affinity for Pb partly because thesoil pH (5.6 to 6.2) is below the zero point of charge (pH 8.5) of Fe-oxides(Dragun, 1988); a positive surface charge precludes cation exchange reactionsinvolving Pb. Elliott et al. (1986) noted that removal of Fe-oxides from aPlinthic Paleudult reduced the soil ZPC from 6.0 to 2.3 and increased thesorption of heavy metals. Another contributing factor may be the predomi-

11


nance of hematite and (or) goethite in the Ultisol studied. Well-crystallized Fe-oxides are thought to have less of an affinity for Pb than freshly precipitatedand less crystalline forms (Dragun, 1988), and Aualiitia and Pickering (1987)have shown that specific sorption by hematitic and goethitic iron ores is limitedat pH values less than about 6.0.

Table 3 also compares the summed total determined by sequential extractionwith the total obtained using the bulk sample. The comparison shows that thesummed total sorbed is always less than the bulk total sorbed. This is probablypartly the result of the inevitable loss of a small amount of solid material whenthe samples are washed with deionized water between each step in the sequen-tial extraction procedure, and partly due to random sample variability. Thedifferences range from 4.6 to 64.6% (average of 30.2% with a standard devia-tion of 15.9%). These differences are somewhat greater than those of 20%reported by Harrison et al. (1981). It is also evident from the results in Table3 that the proportions of the Pb sorbed by each individual phase are more orless the same despite the range in concentrations used. Hence, a better estima-tion of the actual amounts of Pb in each targeted phase could theoretically beobtained using the relative proportions in each phase and the bulk totals as acorrection factor.

Table 4 indicates that the quantities of Pb sorbed by each of the bulk samplesfor both soils decrease with increasing spike concentrations, as expected. Thedecrease of the sorption is probably because there are only a limited numberof sorption sites in the soils for this particular metal. The Alfisol B horizonstands out as having sorbed almost all of the Pb added up to a spiking solutionconcentration of 3000 mg/l. Carbonate is clearly of prime importance in con-trolling the Pb in the soils studied. Lime also has been recognized for many

TABLE 4Quantities of Lead Sorbed by Bulk Soil Samples

Sample type

SpikeAlfisol Ultisol

concentrationA horizon B horizon A horizon B horizon

(mg/l) mg/kg % a mg/kg % mg/kg % mg/kg %

500 4,724.8 59.1 7,988.2 99.8 3,099.2 38.7 3,643.2 45.51,000 6,036.8 37.7 15,954.2 99.7 3,760.0 23.5 4,523.2 28.33,000 11,035.2 23.0 47,520.0 99.0 6,336.0 13.2 8,011.2 16.74,000 7,252.8 11.3 52,272.0 81.7 5,472.0 8.6 6,720.0 10.55,000 9,172.8 11.5 54,980.8 68.7 7,792.0 9.7 8,452.8 10.66,000 9,595.2 10.0 52,811.2 55.0 10,576.0 11.0 9,883.2 10.38,000 10,395.2 8.1 59,280.0 46.3 4,448.0 3.5 6,016.0 4.7

a Percentage of total added in spiking solution.

12


years as an excellent immobilizing agent for the remediation of sites contami-nated by heavy metals (John, 1972; Korte et al., 1976).

D. Sorption Isotherms of Bulk Samples

Figure 1 shows that the Alfisol B horizon is characterized by an H-type isotherm,whereas the other sample isotherms are of the L-type. The H-type isotherm isusually produced by either significant van der Waal’s interactions contributing tothe sorption process, or by a highly specific interaction between the solid phase andthe sorbing substance (Dragun, 1988). The results of sequential extraction analysis(Table 3) suggest that the carbonate phase is responsible for the strong affinity ofthe Alfisol B horizon for Pb. McBride (1979) has shown that carbonate sorbs Mnby a combination of precipitation and specific adsorption; presumbly, the samerelations are true for Pb. L-type isotherms reflect a strong affinity at low concen-trations, but less affinity at higher levels due to a limiting number of sorption sitesand a decrease in sorbing surface as the excess of sorbate increases (Sposito, 1984).This isotherm shape is thought to be produced when physisorption or ion exchangeis dominant, and is characteristic of layer silicate minerals and organic matter(Farrah et al., 1980; Adriano, 1986). Thus, the observed isotherm shapes (Figure 1)concur with sequential extraction results (Table 3), indicating the importance ofclay content and (or) organic matter in controlling Pb sorption in the Alfisol Ahorizon and in both Ultisol horizons.

E. Sorption Isotherms of Individual Fractions

Using CSSA, Salim et al. (1993) showed that it is possible to construct isothermsnot only for bulk sediment samples, but for individual phases as well. This is done

FIGURE 1. Lead sorption isotherms for bulk samples. Plot at right corresponds to that atleft but at expanded scale.

13


FIGURE 2. Lead sorption isotherms for each form targeted by sequential extractionanalysis. Plots at right correspond to those at left but at expanded scale.

by plotting the amounts recovered in each step of the sequential extraction proce-dure after spiking against the corresponding equilibrium solution concentrationdetermined using the bulk sample. This approach has a definite advantage overprevious methods, which have relied on bulk samples or single pure phases inisolation, because it allows sorption capacities to be determined for individualphases while together in a natural mixture. Figures 2 and 3 show that it is alsopossible to build isotherms successfully for most of the individual fractions exam-ined in this study. Correlation coefficients indicate that there is a good fit to theFreundlich equation (Table 5) in many cases. The poorest fit generally seems to beassociated with the oxide-occluded fractions, similar to the findings of Salim et al.(1993). This problem is perhaps attributable to the small quantities of oxidespresent, thereby resulting in greater scatter among samples.

The shapes of the isotherms for individual fractions are similar to those of thecorresponding bulk samples. Thus, with the exception of the H-type isotherm

14


FIGURE 3. Lead sorption isotherms for each form targeted by sequential extractionanalysis. Plots at right correspond to those at left but at expanded scale.

derived for the carbonate fraction of the Alfisol B horizon, the isotherms for theindividual fractions generally appear to be of the L-type. As noted previously, theshapes of isotherms are thought to reflect the type of sorption mechanism takingplace, with H-type and L-type isotherms representing chemisorption andphysisorption, respectively (Sposito, 1984; Dragun, 1988). However, care must beexercised when interpreting and comparing isotherm shapes because it is notpossible to distinguish sorption and precipitation using isotherm data alone (Sposito,1984), and because the isotherm shapes depend on such factors as the scales usedin plotting and the relative proportions of phases in the samples. To the extent thatsorption mechanisms are reflected in the shapes of the isotherms in Figures 2 and3, there is no indication of specific adsorption by oxides, even in the Ultisol Bhorizon.

Table 5 shows that there is an excellent fit between the data and the particularform of the Langmuir equation used; hence, estimated sorption capacities wereobtained not only for bulk samples, but for individual phases as well (Table 6). The

15


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16


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17


total sorption capacities obtained by summing the capacities of each individualfraction show good agreement with those measured directly using bulk samples.This successful mass balance implies that the individual capacities are reasonablyaccurate. The differences in sorption capacities (summed total vs. bulk total)average about 23%, comparable to results reported by Salim et al. (1993) forMichigan glacial lake sediments. Except for the Alfisol B horizon, the sorptioncapacity of the exchangeable fraction is greatest, and it is clear that the capacitiesof the other fractions have been exceeded after spiking. This accounts for theobserved predominance of exchangeable forms of Pb under conditions simulatingsevere contamination. Farrah et al. (1980) obtained the adsorption capacities ofillite, kaolinite, and montmorillonite for Pb using “pure” samples. With their data,and the proportions of clay and clay mineral types in the Alfisol B horizon(Table 1), an approximate adsorption capacity of about 5600 mg/kg is calculatedfor the exchangeable fraction using montmorillonite to conservatively estimate theadsorption capacity of vermiculite. Because the observed capacity is only 526 mg/kg(Table 6), this result suggests that an evaluation of the attenuation capability of apart of a soil based on “pure” phases may be overestimated by an order ofmagnitude or more.

The bulk sorption capacity of the Alfisol A horizon for Pb (10,437 mg/kg) issomewhat lower than the capacities of other Michigan soils (10,712 to 15,656 mg/kg)characterized by Zimdahl and Skogerboe (1977), and of Alfisols in Pennsylvaniaand New York (14,248 mg/kg and 25,792 mg/kg, respectively) reported previouslyby Harter (1979). The B horizon of the Alfisol studied stands out as having thecapacity to sorb the greatest levels of Pb. This is obviously due to the carbonatefraction, which itself is able to occlude 46,168 mg/kg of the Pb, or about 90% ofthe summed total. However, an even higher sorption capacity (180,500 mg/kg) wasobtained by Salim et al. (1993) for calcareous sediments containing about 25%carbonate (Table 1). The sorption capacities obtained for the Ultisol studied (5453to 7008 mg/kg) are within the range of those reported previously (Harter, 1979) forUltisols in Maryland and Pennsylvania (4536 mg/kg and 16,432 mg/kg, respec-tively).

V. SUMMARY AND CONCLUSIONS

The behavior of Pb in the A and B horizons of a Michigan Alfisol and a VirginiaUltisol was studied by CSSA after spiking to simulate severe contamination. Allof the samples sorbed large quantities (~3000 to 60,000 mg/kg) of Pb. CSSAshowed that Pb is predominantly in an exchangeable form in the A horizon of theAlfisol and in both horizons of the Ultisol because the sorption capacities of thephases commonly responsible for the fixation of Pb at low to moderate levels ofcontamination are exceeded. Carbonate sorbed the bulk of the Pb in the Alfisol Bhorizon and has a high sorption capacity in both soils, despite the presence of other

18


phases with strong affinities for Pb. The exchangeable form is mobile andbioavailable (Luoma, 1983; Gunn et al., 1988; Dragun, 1988). Hence, these resultssuggest the following scenarios in the event of severe Pb contamination: (1) the Ahorizons of both soils are expected to pose a serious toxic hazard to humans eitherdirectly by ingestion or indirectly because of bioaccumulation in the food chain,(2) groundwater contamination is possible in association with the Ultisol becausePb in the B horizon is mobile, and (3) no groundwater contamination is expectedin association with the Alfisol because Pb in the B horizon is immobilized byfixation reactions involving carbonate precipitation. Although it was anticipatedthat significant quantities of exchangeable Pb would be present at high levels ofcontamination, the magnitudes observed were unexpected because at low to mod-erate levels of contamination, Pb is commonly immobilized in soils by fixationreactions involving organic matter and oxide minerals.

In recent years, there has been great interest in the effects of heavy metals resultingfrom the land disposal of untreated wastes such as sewage sludge, municipal waste-water, landfill leachates, dredged sediments, and oil refinery wastes. Hence, manyinvestigations have been carried out to determine which properties are important forpredicting the attenuation capabilities of soils and for regulating the surface applica-tions of such wastes. These studies have relied on samples of whole soils or ofindividual components in isolation. The former approach is limited by the fact thatno information is gained regarding the actual partitioning of metals in various forms;the latter by the fact that interactions among phases cannot be documented. TheCSSA approach is superior because it overcomes both of these limitations; however,further studies are needed. Only shock loadings by Pb were investigated in this study,and the geochemical behavior of Pb may not necessarily be the same in the presenceof other metals or if high levels are achieved by a gradual buildup through lowloading levels over a prolonged period of time. The results may be applicable atheavily contaminated sites where rapid loading probably does occur, such as shoot-ing ranges and hazardous waste dump sites.

ACKNOWLEDGMENTS

Thanks to W. L. Daniels and M. Genther of the Department of Crop and SoilEnvironmental Sciences at VPI&SU, Blacksburg, VA, for supplying the Ultisolsamples and characterization data; and to the Michigan Mineralogical Society forpartial funding of the project.

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geochemical behavior of lead in an alfisol and an ultisol ... · pdf filean alfisol was...

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